Monolithic integration of iii-v cells for powering memory erasure devices

ABSTRACT

A method for making a photovoltaic device is provided that includes the steps of providing a silicon substrate having a complementary metal-oxide semiconductor (“CMOS”); bonding a first layer of silicon oxide to a second layer of silicon oxide wherein the bonded layers are deposited on the silicon substrate; and forming a III-V photovoltaic cell on a side of the bonded silicon oxide layers opposite the silicon substrate, wherein when the III-V photovoltaic cell is exposed to radiation, the III-V photovoltaic cell generates a current that powers a memory erasure device to cause an alteration of a memory state of a memory cell in an integrated circuit.

DOMESTIC PRIORITY

This application is a continuation of Non-Provisional application Ser.No. 15/725,742, entitled “MONOLITHIC INTEGRATION OF III-V CELLS FORPOWERING MEMORY ERASURE DEVICES”, filed Oct. 5, 2017, which is adivisional of Non-Provisional application Ser. No. 14/994,267, entitled“MONOLITHIC INTEGRATION OF III-V CELLS FOR POWERING MEMORY ERASUREDEVICES”, filed Jan. 13, 2016, now U.S. Pat. No. 9,997,475, issued Jun.12, 2018, the contents of which are incorporated by reference.

BACKGROUND

The present invention generally relates to photovoltaic solar cells, andmore specifically, to integration of III-V cells for powering memoryerasure devices.

A photovoltaic device is a device that converts the energy of incidentphotons to electrical power. Typical photovoltaic devices include solarcells, which are configured to convert the energy in the electromagneticradiation from the Sun to electric energy. Each photon has an energygiven by the formula E=hv, in which the energy E is equal to the productof the Planck's constant h and the frequency v of the electromagneticradiation associated with the photon.

Hardware based “Root of Trust” is a fundamental building block for anysecure computing system. Key elements of secure computing requireauthentication, sending data to an authorized source, and/or loadingdata onto a designated device. In general, cryptographic keys in binarycode form the basis of securing data and bit streams. Typically, suchcryptographic keys are stored in non-volatile memory and are present onan integrated circuit (IC) at all times. If an attacker can extract thekey from a device, the entire foundation for secure computing is injeopardy. For example, an attacker with physical access to a device candelayer the chip and read out the stored code based on the state of thetransistors. Thus, securing cryptographic keys requires anti-tampertechnologies. For example, an anti-tamper mesh may surround a printedcircuit board and may include a tamper sensor chip and its own batterypack to deter such attacks. If the sensor detects that the mesh is beingcut, the cryptographic code is erased. However, such anti-tampertechnologies may be relatively expensive and may therefore not besuitable for implementation in mass produced, cost sensitive deviceslike field programmable gate arrays (FPGAs), mobile devices, andsensors.

SUMMARY

In an embodiment of the invention, a method for making a photovoltaicdevice is provided that includes the steps of providing a siliconsubstrate having a complementary metal-oxide semiconductor (“CMOS”);bonding a first layer of silicon oxide to a second layer of siliconoxide wherein the bonded layers are deposited on the silicon substrate;and forming a III-V photovoltaic cell on a side of the bonded siliconoxide layers opposite the silicon substrate, wherein when the III-Vphotovoltaic cell is exposed to radiation, the III-V photovoltaic cellgenerates a current that powers a memory erasure device to cause analteration of a memory state of a memory cell in an integrated circuit.

In another embodiment of the invention, a method for making aphotovoltaic device is provided that includes the steps oflow-temperature bonding of a III-V wafer containing an extremely thinepitaxial template layer; selective removal of a sacrificial releaselayer or full removal of a III-V substrate; deposition and patterningoxide trenches; selective epitaxial growth of a III-V photovoltaic cell;and planarization and deposition of a transparent conducting layer.

In another embodiment of the invention, a photovoltaic device isprovided that includes a silicon substrate comprising a complementarymetal-oxide semiconductor (“CMOS”); a first layer of silicon oxidebonded to a second layer of silicon oxide wherein the bonded layers aredeposited on the silicon substrate; and a III-V photovoltaic cell on aside of the bonded silicon oxide layers opposite the silicon substrate,wherein when the III-V photovoltaic cell is exposed to radiation, theIII-V photovoltaic cell generates a current that powers a memory erasuredevice to cause an alteration of a memory state of a memory cell in anintegrated circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter which is regarded as the invention is particularlypointed out and distinctly claimed in the claims at the conclusion ofthe specification. The forgoing and other features, and advantages ofthe invention are apparent from the following detailed description takenin conjunction with the accompanying drawings in which:

FIG. 1 shows a schematic of an embodiment of an inverted single-junctionGaAs solar cell and the summary of the examined embedded sacrificiallayers;

FIG. 2 shows a schematic of an embodiment of an upright single-junctionGaAs solar cell;

FIG. 3 shows a schematic illustration of the photovoltaic integrationwith silicon involving (a) low-temperature bonding of the photovoltaiconto silicon followed by (b) selective removal of the sacrificialrelease layer, resulting in (c) thin-film photovoltaic on silicon; and

FIG. 4 shows a schematic illustration of the photovoltaic integrationwith silicon involving (a) low-temperature bonding of a III-V wafercontaining an extremely thin epitaxial template layer, (b) selectiveremoval of the sacrificial release layer or full removal of the III-Vsubstrate, (c) deposition and patterning oxide trenches, (d) selectiveepitaxial growth of the PV cell, and (e) planarization and deposition ofthe transparent conducting layer (ITO, ZnO, graphene, CNTs). (f)Exemplary structure showing a JFET and a depletion-mode FET fabricatedon the InP epitaxial template layer.

DETAILED DESCRIPTION

Embodiments of the present disclosure present a novel on-chip antitamper device for detecting physical tampering as well as for providinga tamper response by erasure of data. Exemplary components includeminiaturized photovoltaic cells (PV) integrated into the back end of theline (or backend) interconnect structure at various levels, non-volatilememory (NVM) (e.g., phase change memory (PCM)) to store sensitive data,such as secret authentication codes, and a memory erasure device (MED).In one embodiment, the MED comprises an embedded reactive material,which may comprise a multilayer thin film metal stack that reacts whentriggered by a current pulse generated from the photovoltaic (PV)elements. Notably, many physical reverse engineering techniques requireaccessing the chip structures through imaging (e.g., electron beams fromscanning electron microscopy (SEM), focused ion beam (FIB), x-ray, etc.)and therefore generate radiation (e.g., photocurrent, laser beam inducedcurrent (LBIC), electron beam induced current (EBIC), and the like).Embodiments of the present disclosure exploit this principle by usingthe miniature photovoltaic cells to convert the radiation from atampering attempt into a current which triggers the tamper response(e.g., either directly or through an exothermic reaction generated inthe reactive material) to erase the data.

Tampering/attacks range from electrical probing and delayering forextracting secret keys to inducing faults (e.g. flipping states) inorder to force an integrated circuit to conduct unauthorized operations.An attacker typically needs to deploy a range of techniques in order tolocate specific circuits and structures which usually involve radiationfor imaging or inducing currents and faults. Table 1 illustrates severalfailure analysis/tampering techniques.

TABLE 1 Failure Analysis technique Radiation involved Delayering byashing, polishing Requires optical microscope to stop at a specificlayer Single and dual beam focus ion Ga ion beam for FIB cuts (producebeam secondary electrons) Scanning electron microscope (e- beam up to 30KeV) Electron beam induced current Scanning electron microscope (e-(EBIC), electron beam probing beam up to 30 KeV) Laser techniques: laservoltage Lasers wavelength up to 1.3 probing, optical beam inducedmicrons current (OBIC), thermal e.g. optical beam induced resistancechange (OBIRCH) X-Ray tomography X-Ray ranging from 50-200 keV

Embodiments of the present disclosure are compatible with standard metaloxide semiconductor (MOS) chip fabrication techniques, thereby reducingthe cost per die during implementation and integration of suchembodiments. Furthermore, the security against physical tampering isincreased compared to packaging based approaches due to miniaturizationand containment within the chip. In addition, the on-chip anti-tamperdevices of the present disclosure may also be implemented withestablished hardware security products.

As stated above, the present invention relates to photovoltaic solarcells, and particularly to integration of III-V cells for poweringmemory erasure devices, which are now described in detail withaccompanying figures. These novel integrated cells provide excellentanti-tampering devices. Further the III-V cells can be tuned to variouswavelengths of radiation. It is noted that like reference numerals referto like elements across different embodiments.

As used herein, the terms “invention” or “present invention” arenon-limiting terms and not intended to refer to any single aspect of theparticular invention but encompass all possible aspects as described inthe specification and the claims.

As used herein, the term “about” modifying the quantity of aningredient, component, or reactant of the invention employed refers tovariation in the numerical quantity that can occur, for example, throughtypical measuring and liquid handling procedures used for makingconcentrates or solutions. Furthermore, variation can occur frominadvertent error in measuring procedures, differences in themanufacture, source, or purity of the ingredients employed to make thecompositions or carry out the methods, and the like. In one aspect, theterm “about” means within 10% of the reported numerical value. Inanother aspect, the term “about” means within 5% of the reportednumerical value. Yet, in another aspect, the term “about” means within10, 9, 8, 7, 6, 5, 4, 3, 2, or 1% of the reported numerical value.

As used herein, a “photovoltaic device” is a device, such as a solarcell, that produces free electrons-hole pairs, when exposed toradiation, such as light, and can be collected to produce an electriccurrent. The photovoltaic device typically includes layers of p-typeconductivity and n-type conductivity that share an interface to providea junction. The “absorption layer” of the photovoltaic device is thematerial that readily absorbs photons to generate charge carriers, i.e.,free electrons or holes. A portion of the photovoltaic device, betweenthe front side and the junction is referred to as the “buffer layer”,and the junction is referred to as the “buffer junction.” The bufferlayer may be present atop the absorption layer, in which the bufferlayer has a conductivity type that is opposite the conductivity type asthe absorption layer. In one example, when the Sun's energy in the formof photons collects in the cell layers, electron-hole pairs aregenerated in the material within the photovoltaic device. The bufferjunction provides the required electric field for the collection of thephoto-generated holes and electrons on the p-doped and n-doped sides ofthe buffer junction, respectively. In many examples, at least one p-typelayer of the photovoltaic device may provide the absorption layer, andat least one adjacent n-type layer may provide the buffer layer.

While some embodiments of the present disclosure may employ other typesof non-volatile memory, phase change memory has particular advantageswhich make it a suitable choice for use in a secure device according tothe present disclosure. More specifically, from a physical anti-tamperperspective phase change memory does not give off any electromagneticsignature. Therefore, reading out the memory states can only beaccomplished by direct probing and imaging, which requires physicalaccess to the phase change memory. Once a reactive material has ignitedand melted the phase change memory, there is no way to read out the bitssince the physical attributes of the material have been changed. This isdifferent from SRAM and other technologies, where it is possible to readout the last state due to imprinting, even after the chip power isturned off. On the other hand, metal oxide semiconductor based memorycells may be advantageous in some application because metals, such asaluminum (which may be used in exemplary reactive materials), causesignificant reactions with dielectrics such as silicon dioxide, siliconnitride, and high-K gate dielectrics, reacting with oxygen and formingstable oxides. Thus, there is significant potential for extensivegate/memory cell destruction when such materials are used.

In one embodiment the MED comprises a reactive material. An exothermicreaction of the reactive material is triggered by a current from aphotovoltaic cell. Here, the inventive photovoltaic solar cell comprisesa III-V cell structure. As used herein “III” refers to the IUPAC Group13 class of elements on the periodic table that is also known as theBoron group. Group III is a historical name of this class of elementscharacterized by having three electrons in their outer energy levels(valence layers). As used herein, Group III elements comprise boron (B),aluminum (Al), gallium (Ga), indium (In), thallium (Tl), and ununtrium(Uut).

As used herein “V” refers to the IUPAC Group 15 class of elements on theperiodic table that is also known as the Nitrogen group. The “five”(“V”) in the historical name comes from the “pentavalency” of nitrogen,reflected by the stoichiometry of compounds such as N₂O₅. As usedherein, Group V elements comprise nitrogen (N), phosphorus (P), arsenic(As), antimony (Sb), bismuth (Bi) and the synthetic element ununpentium(Uup) (unconfirmed).

Compound semiconductors are attractive for co-integration with acomplementary metal-oxide semiconductor (“CMOS”) owing to theirpotentially attainable high conversion efficiency while utilizing areasonably thin absorber layer. However, the direct growth ofhigh-quality compound semiconductors on silicon has long beenchallenging. Here, the instant method to integrate III-V cells with CMOSis to create thin-film devices utilizing a suitable layer transfermethod and subsequently attach a surrogate CMOS chip. In one embodiment,the method comprises a variant of epitaxial layer lift-off (ELO) thatinvolves the selective removal of a sacrificial layer, which is embeddedbetween the device layer and the host substrate.

In one embodiment the solar cell comprises a single-junction GaAs cellswith an embedded sacrificial layer. Single-junction GaAs solar celldevices are theoretically predicted to achieve the highest conversionefficiency. In one embodiment of a process for integrating the solarcell device with the CMOS chip, an inverted GaAs device structure isgrown. Two different release layers, InGaP and InAlP, were used forproducing thin-film GaAs solar cells. FIG. 1 schematically illustratesthe device structure for an inverted GaAs solar cell 100 with anembedded sacrificial layer. The table in FIG. 1 additionally providesthe processing details for the examined sacrificial layers, in which thegrowth temperature and the thickness of the sacrificial layer werevaried while the growth condition of the solar cell was identical in allexperiments.

Two structures were initially grown utilizing lattice-matched InAlPfilms as the release layers. The solar cell devices were grown underidentical conditions with the exception of the growth temperature forthe InAlP layers. After the epitaxial growth of the structures, thesamples were immersed in a concentrated hydrochloric acid (HCl). It wasfound that the InAlP layer grown at the lower temperature of 640° C. wasfully etched in the concentrated HCl solution, resulting in the releaseof the device structure. However, the sample consisting of thehigh-temperature InAlP layer (680° C.) remained unchanged upon theexposure to the HCl solution. Furthermore, an additional solar cellstructure was grown utilizing a lattice-matched InGaP sacrificial layerand subsequently immersed in the HCl solution. However, the devices withthe InGaP layer did not separate from the host GaAs substrate upon theirimmersion in the solution for elongated times in excess of 12 hrs. Ashort-loop device fabrication process was additionally devised aiming toquickly evaluate the electrical properties of different solar celldevices. The open circuit voltage (V_(∝)) was used as the key figure ofmerit for evaluating the epitaxial quality of the cells. The V_(∝) wasexamined primarily because of its extreme sensitivity to the epitaxialdefects that give rise to an increased dark current. The V_(∝) of allthree structures was found to be in excess of 900 mV, indicating thegood quality of the epitaxial films.

In another embodiment, the solar cell comprises a single junction InGaAssolar cell tuned for the infrared (“IR”) spectrum. The narrow energybandgap of InGaAs extends the cutoff absorption wavelength of the solarcells to about 1630 nm. The device structure was grown on InP substratesbecause of its matched lattice constant to that of InP. Once the devicestructure is grown, the substrate is subsequently removed selectivelywith respect to the InGaAs in order to produce thin-film solar cells.FIG. 2 shows the schematic illustration of the InGaAs solar cell 110,grown using metal oxide chemical vapor deposition (“MOCVD”). Thepreliminary results indicate the successful removal of the InP substratewith respect to the solar cell structure. However, further calibrationshould mitigate the observed anomalous wafer-to-wafer variability.Nevertheless, the preliminary results from the ‘best’ solar cell devicesare promising, where the V_(∝) of the best cells was found to be inexcess of 420 mV. The development of an inverted structure will furtherallow the subsequent transfer and integration with the surrogate CMOSchip.

In one embodiment, the III-V cells are attached to a surrogate siliconsubstrate. As shown in FIG. 3, a silicon substrate 120 is provided. Inone embodiment, the silicon substrate 120 comprises a CMOS chip. A firstsilicon oxide layer 130 is provided on top of the silicon substrate. Asecond silicon oxide layer 140 is bonded to the first silicon oxidelayer 130. An inverted III-V photovoltaic cell 150 is provided on a sideof the bonded silicon oxide layers 130, 140 opposite the siliconsubstrate 120. In one embodiment, the inverted III-V photovoltaic cell150 comprises GaAs. In one embodiment, the inverted III-V photovoltaiccell 150 is in electrical contact with the CMOS. Such contact can madethrough external wiring or internally through vias, traces, and thelike. A release layer 160 is provided on a side of the inverted III-Vphotovoltaic cell 150 opposite the bonded silicon oxide layers 130, 140.A substrate “handler” layer 170 is provided on a side of the releaselayer 160 opposite the inverted III-V photovoltaic cell 150.

FIG. 3 schematically illustrates one exemplary method to obtainthin-film photovoltaic cell on silicon. This embodiment comprises thestep of a low-temperature plasma assisted bonding of the invertedphotovoltaic devices 150 onto an oxide-coated silicon 120, 130, 140.This embodiment further comprises the step of releasing the invertedIII-V photovoltaic device 150 using the ELO process. The release layer160 is etched and removed. The substrate layer is mechanically removed170. In one embodiment, the bonding process results in a thin bond line,preferably atomically thin. In another embodiment, in addition to theoxide-oxide bonding scheme, a eutectic metal bonding approach is used aswell to allow the formation of the metal electrodes on the oppositesides of the device, which could lead to a potentially simplerintegration scheme.

In another embodiment, an alternative integration scheme and devicestructure is disclosed, which tends to facilitate the integration ofInGaAs photovoltaic cells on silicon. In the case of a GaAs photovoltaicdevice, the thin InP layer can be replaced by either thin Ge or otherIII-V cells that are lattice-matched with GaAs. The process starts withthe bonding of an extremely thin (about 5 to about 50 nm) heavily dopedInP layer 180 on silicon 120, 130, 140. The thin InP layer 180 is formedusing similar process flow described above. The process involves thelow-temperature bonding of a III-V substrate containing the thin InPlayer 180 onto a silicon wafer 120, 130, 140 followed by the ELO processor the full removal of the substrate 170 in order to leave behind thethin InP layer on silicon 120, 130, 140. The thin InP layer 180 providesseveral new device design possibilities including universal back accessfor connecting small photovoltaic cells that are distributed on thechip, fabrication of active switches such as junction gate field-effecttransistors (“JFETs”) or depletion mode FETs that are located betweenthe MEDs and the photovoltaic cells, etc.

FIG. 4 schematically illustrates the process and some of the exemplarystructures that can be achieved using this new process. A siliconsubstrate 120 is provided. In one embodiment, the silicon substrate 120comprises a CMOS chip. A first silicon oxide layer 130 is provided ontop of the silicon substrate. A second silicon oxide layer 140 is bondedto the first silicon oxide layer 130. A thin InP layer 180 is providedon a side of the bonded silicon oxide layers 130, 140 opposite thesilicon substrate 120. A release layer 170 is provided on a side of thethin InP layer 180 opposite the bonded silicon oxide layers 130, 140. AnInP substrate “handler” layer 170 is provided on a side of the releaselayer 160 opposite the thin InP layer 180.

Still referring to FIG. 4, the release layer 160 and InP substrate 170are removed exposing the thin InP layer 180 on the silicon 120, 130,140. Three columns of silicon oxide 190 are deposited on a side of thethin InP layer 180 opposite the bonded silicon oxide layers 130, 140thereby forming two wells. III-V photovoltaic cells 200 are selectivelyepitaxially grown within the wells. A transparent conductive layer 210is deposited on a side of the III-V photovoltaic cells 200 and siliconcolumns 190 opposite the thin InP layer 180. In an embodiment, thetransparent conductive layer 210 comprises a transparent conductiveoxide.

FIG. 4 also shows an exemplary structure that further comprises a memorycell 215, a memory erasure device 220, control circuitry 230, and adoped amorphous silica high K metal gate 240.

As disclosed herein, the methods produce monolithic single chip devicesthus simplifying the manufacturing process.

In some embodiments, multiple photovoltaic cells are included at variouslayers and at various locations, random or otherwise, in the backend, orfront end, of the integrated circuit. As such, different photovoltaiccells included in the integrated circuit may be formed at differenttimes as each of the layers of the integrated circuit is fabricated. Inthis regard, it should be noted that any one or more of the steps of themethod may be repeated or performed in a different order than asexplicitly depicted in FIG. 3 and FIG. 4. In some embodiments, multiplephotovoltaic cells are formed having different sizes, geometries,configurations or materials/compositions, resulting in differentresponse from one another in response to various tampering techniques(e.g., different quantum efficiencies when exposed to the samewavelength radiation). In some embodiments, the reactive material andthe photovoltaic cell may be electrically coupled through one or morevias and/or traces, and the like.

The photovoltaic cell, or the photovoltaic cell in conjunction with thereactive material, comprise a tamper response device for the integratedcircuit. Radiation from tampering/probing techniques that is received atthe photovoltaic cell generates a photocurrent from the photovoltaiccell to the reactive material or directly to the memory cell. The directphotocurrent, or heat from an exothermic reaction in the reactivematerial erases or changes a state of the memory cell, preventingextraction of any data previously stored in the memory cell.

In some embodiments, the reactive material is included in an integratedcircuit which provides a way to store the required amount of energy forirreversible erasure of memory cells on the chip through an exothermicreaction. In particular, in many cases the radiation of a given tamperevent may be insufficient to generate a large enough current pulse fromone or more integrated photovoltaic cells to directly alter, e.g., eraseor destroy, the non-volatile memory, whereas the reactive material mayhave a steep chemical gradient and require only an ultra-low triggercurrent to release the energy. In one embodiment, the exothermicreaction irreversibly destroys all or a portion of a non-volatile memoryproviding unmistakable evidence of tampering. In one embodiment, thereactive material is deposited in at least one continuous layer adjacentto the non-volatile memory or sufficiently close to alter thenon-volatile memory, e.g., to change the memory state of thenon-volatile memory or to damage or destroy it such that it is no longeruseable as a memory.

In some embodiments, the reactive material comprises a thin metal filmstack of several metallic layers. For example, the reactive materialcomprises one or more specific metals such as copper, copper oxide,aluminum, nickel, hafnium oxide, silicon, boron, titanium, cobalt,palladium, platinum, and/or other metallic structures. In oneembodiment, the stack includes an aluminum layer sandwiched between twonickel layers. Advantageously, the reactive material stores energyon-chip that is benign during normal chip operations, but which can betriggered by a low current pulse. This is contrary to traditional tamperschemes with on-chip batteries requiring a constant power since thetamper detection and response circuitry has to be kept operationalthroughout the lifetime of the die. In addition, current based ignitionis potentially only needed on a single site/contact of the reactivematerial. In particular, a reaction self-propagates in free standingfoils and similarly will sustain a self-propagating reaction front onthe chip. Furthermore, if the heat loss away from the reactive materialto the surrounding chip environment quenches the reaction, bit erasurecan still be accomplished by exothermic reaction in reactive materialregions exposed to current flows. The current to drive the reactionwithout a self-sustaining reaction front is still lower the currentrequired for direct bit erasure. For example, ignition current may besubstantially lower than that required to directly reset thenon-volatile memory (e.g., nano-amperes versus micro-amperes).

In various embodiments, the ignition temperature and heat of reactionare tuned/adjusted to requirements of the particular manufacturingenvironment. In particular, the reactive material is inert duringprocessing and survive normal chip operation and stress tests, but issensitive enough to ignite during tampering and provide sufficient heatto destroy or erase non-volatile memory cells. It has been shown, forexample, that in a titanium/amorphous-silicon stack, with a bilayerspacing of 75 nm deposited on 1 μm of silicon oxide on silicon, thereaction quenches when the reactive material thickness is less than 2.25μm. Thus, in some embodiments the potential for reaction quenching istaken into account in providing a reactive material that generates asufficient reaction front that will not be quenched prior to destroyingthe non-volatile memory. In some cases, the ignition threshold and heatof the reaction is varied by changing of the chemistry, stoichiometry,and microstructure (grain structure and line spacing in thin filmscomprising the stack of reactive material). For instance, the number oflayers, the spacing between layers, the width of the layers and thecomponent metals in the stack of reactive material, can all be varied,resulting in different ignition fluences (current densities required forignition), different reaction heats, etc. In some embodiments, theignition current density can also be varied via lithography, such as byvarying of the size of the electrical contacts/vias connected to thestack of reactive material. For example, a connection from a firstphotovoltaic cell may be fabricated with electrical contacts having adifferent cross-sectional area than electrical contacts for a connectionfrom a second photovoltaic cell. Accordingly, the current densitiesdelivered from the first and second photovoltaic cells may be differentfrom one another despite, for example, the first and second photovoltaiccells being otherwise the same and receiving the same incidentradiation. Similarly, in embodiments employing oxide systems, suchoxides can be made porous by producing nanoparticles of different sizes.Likewise, the potential for reaction quenching can be reduced byfabricating specific patterns and geometries for the reactive material,or one or more layers thereof, through lithography, undercutting,chemical mechanical planarization, and other similar techniques, inorder to promote self-propagation by reducing heat loss. For example, amultilayer, such as nickel-aluminum Ni/Al, can be grown by sputteringand evaporation. In any case, when the reactive material is triggered bya current pulse, the stack of reactive material reacts by spontaneousmixing to release a large quantity of heat through an exothermicreaction, without any pressure waves or gaseous byproducts.

In one embodiment, nickel/aluminum multilayers are used for the stack ofreactive material, since such a reactive material can be ignited using ashort, low-current electrical pulse. For example, a bilayer spacing isfrequently used to characterize the microstructure of a multilayerreactive material. For Ni/Al multilayers, the bilayer spacing is definedas the thickness of one layer of nickel and one layer of aluminum. Asone example, assuming a Ni/Al multilayer with a density of 5.09 g/cc, aheat capacity of 0.588 J/g/K, a resistivity of 1.3×10⁻⁵ Ω-cm and a 51 nmbilayer spacing, the ignition temperature is 177° C. In addition,assuming that 50 nm electrodes are used to supply an ignition current, a100 ns 1.5 pA pulse is sufficient, assuming a lossless environment.However, estimating 80% heat loss, an approximately 3.3 pA pulse of 100ns will ignite the Ni/Al (since electrical energy scales as the squareof the current). Notably, the reaction of the reactive material tovarious currents of various durations can be tuned by changing the filmthickness, bilayer spacing and chemistry, as well as throughlithographic variation of the contact area to the reactive materialstructure, which will vary the current density.

In addition to Ni+Al multilayers, suitable reactive material componentsand combinations include: Si+2B, Cu+Pd, Al+Ti, Si+Co, Ni+Ti, Co+Al,Al+Pt, and combination of the foregoing as well as Al₃Ni₂, halfniumoxide, copper, copper oxide and various other metal and/or metal oxidematerial platforms already used in metal oxide semiconductormanufacturing. It should be noted that the foregoing is provided by wayof example only and not limitation. Thus, numerous other, further anddifferent combinations of component materials may be incorporated into amultilayer reactive material in accordance with various embodiments ofthe present disclosure. Some exemplary reactive materials that may beused in embodiments of the present disclosure are described in “A Surveyof Combustible Metals, Thermites, and Intermettalics for PyrotechnicApplications”, by S. H. Fischer and M. C. Grubelich, Sandia NationalLaboratories, 32^(nd) AIAA/ASME/SAE/ASEE Joint Propulsion Conference,Lake Buena Vista, Fla., 1996, which is incorporated by reference hereinin its entirety. In this regard, it should be noted that although someembodiments describe a reactive material that comprises a multi-layerstack, the present disclosure is not so limited. Namely, in other,further and different embodiments, the reactive material may comprise asingle layer or region of a single metal or other material, or maycomprise a different mechanism for generating heat, such as a thermitemixture, a metal fuel (e.g., a metal oxidation reaction), anintermetallic arrangement other than a multi-layer stack, and the like.In other words, some embodiments may feature an intermetallic reactionwhereas other embodiments may rely upon an exothermicreduction-oxidation reaction, or other mechanism to generate heat.

Reactions in such materials can be ignited by ignition fluencies in therange of, for example, 0.5-5 J/cm², for an approximately 100 μm spotsize, or 5-900 J/cm², for an approximately 10 μm spot size (the size ofelectrical contacts delivering an ignition current). Typicalinterlayer/bilayer spacing is from approximately 2 nm to 200 nm.Reactive materials such as the above can produce heats of reactionranging from approximately 150 calories per cubic centimeter (cal/cc) toapproximately 2500 cal/cc or more. Exemplary multilayer reactivematerials have been show to ignite at temperatures below 300° C. undercertain conditions (e.g., Ni+Al), but can produce heat greater than 600°C. up to more than approximately 2800° C. Given that a phase changememory's amorphous state can be made crystalline at temperatures ofapproximately 400° C. (for an approximately 100 ns pulse), and thatthere are typically greater than 1000 PCM cells on a chip, the energy toignite a reactive material is far less that that required to resistivelyheat each cell individually.

Various embodiments described herein relate to the use of reactivematerials in conjunction with phase change memory. Depending upon theconfiguration of reactive material (e.g., component metal(s)/materials,dimensions, size of contact areas, anticipated tampering techniques, andhence expected ignition currents, etc.), the phase change memory (PCM)can be tuned for optimal use with the reactive material. For example,the doping level can be varied to change the PCM sensitivity to the heatof reaction of the reactive material.

In some embodiments, a reactive material is used in conjunction withCMOS based memory technology. For example, reactive materials maygenerate heat greater than approximately 1300° C., which is sufficientto cause device damage through reactions with dielectrics such assilicon oxide, silicon nitride and high-k/metal gate dielectrics. Forinstance, metals such as aluminum strongly react with oxygen to formstable oxides. In one embodiment, a reactive material is integratedbetween the N and P wells of a CMOS gate. As such, the heat from areaction in the reactive material causes a short due to the destructionof the gate dielectric, causing the circuit element to discharge andthereby irreversibly erasing the state of the memory bit.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, element components,and/or groups thereof.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below are intended toinclude any structure, material, or act for performing the function incombination with other claimed elements as specifically claimed. Thedescription of the present invention has been presented for purposes ofillustration and description, but is not intended to be exhaustive orlimited to the invention in the form disclosed. Many modifications andvariations will be apparent to those of ordinary skill in the artwithout departing from the scope and spirit of the invention. Theembodiment was chosen and described in order to best explain theprinciples of the invention and the practical application, and to enableothers of ordinary skill in the art to understand the invention forvarious embodiments with various modifications as are suited to theparticular use contemplated.

The methods depicted herein are just one example. There may be manyvariations to the steps (or operations) described therein withoutdeparting from the spirit of the invention. For instance, the steps maybe performed in a differing order or steps may be added, deleted ormodified. All of these variations are considered a part of the claimedinvention.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed is:
 1. A photovoltaic device comprising: a III-V wafercomprising an epitaxial template layer bonded thereto; oxide trenchesdefined by oxide columns on the epitaxial template layer, wherein aportion of the epitaxial template layer is exposed; a III-V photovoltaiccell in at least one of the oxide trenches and on the exposed portion ofthe epitaxial template layer; and a transparent conducting layeroverlying the photovoltaic cell and the oxide trenches.
 2. Thephotovoltaic device of claim 1, wherein the III-V photovoltaic cell ison a side of the bonded silicon oxide layers opposite a siliconsubstrate, wherein the photovoltaic device is a monolithic singlecrystal chip.
 3. The photovoltaic device of claim 1 further comprising amemory erasure device connected to the III-V photovoltaic cell, theIII-V photovoltaic cell to generate a current when the III-Vphotovoltaic cell is exposed to radiation, the current to power thememory erasure device to cause an alteration of a memory state of thememory cell.
 4. The photovoltaic device of claim 1, wherein the III-Vphotovoltaic cell comprises a Group III element selected from the groupconsisting of boron (B), aluminum (Al), gallium (Ga), indium (In),thallium (Tl), and ununtrium (Uut) and a Group V element selected fromthe group consisting of nitrogen (N), phosphorus (P), arsenic (As),antimony (Sb), bismuth (Bi) and ununpentium (Uup).
 5. The photovoltaicdevice of claim 3, wherein the memory erasure device comprises areactive material comprising a thin metal film.
 6. The photovoltaicdevice of claim 3, wherein the current generated by the photovoltaiccell triggers an exothermic reaction in a reactive material.
 7. Thephotovoltaic device of claim 6, wherein a heat generated by theexothermic reaction in the reactive material alters the memory state ofthe memory cell.
 8. The photovoltaic device of claim 3, wherein thememory erasure device comprises nickel, aluminum, titanium, copper,palladium, boron, platinum, copper oxide, hafnium oxide, or combinationsthereof.
 9. The photovoltaic device of claim 1, wherein the III-Vphotovoltaic cell comprises a thin III-V layer and the epitaxially grownIII-V layer.
 10. The photovoltaic device of claim 2, wherein the siliconsubstrate comprises a complementary metal-oxide semiconductor.